GitHub actions workflow

[gloskull]

Motivation

• Reduce CI runtime and surface independent failures earlier by parallelizing build and test lanes.
• Improve CI stability and developer experience by adding per-lane caching and cancelling superseded runs.
• Isolate the Python storage-layout check so it does not block the main build/test matrix.

Description

• Reworked .github/workflows/rust.yml to replace the single sequential job with a checks matrix that runs cargo build and cargo test in parallel using matrix entries and per-lane commands like cargo build --locked --verbose and cargo test --locked --verbose.
• Added workflow-level permissions: contents: read and concurrency cancellation to stop superseded runs on the same ref, and set CARGO_INCREMENTAL=0 to disable incremental compilation in CI.
• Added Cargo caching via Swatinem/rust-cache@v2 for registry and build artifacts and a separate storage-layout job that runs python scripts/storage-layout-check.py after updating actions/setup-python to v5.
• Small improvements to step naming and timeouts to make the workflow clearer and more robust.

Testing

• Ran cargo build --locked --verbose which completed successfully (with an unrelated dead-code warning).
• Ran python scripts/storage-layout-check.py which passed with no collisions detected.
• Ran cargo test --locked --verbose which executed the test suite but reported existing failures (17 failing tests) in pool_manager tests due to Soroban storage access outside env.as_contract(); these failures are pre-existing and unrelated to the workflow changes.
• Ran a formatting check (cargo fmt --all -- --check) and observed repository-wide formatting drift, so no fmt gate was added to this workflow change.

Closes #84